1. Field of the Invention
The present invention relates to the release or recovery of subterranean deposits and, more specifically, to system for enhanced recovery of petroleum or other subterranean deposits by injection of steam or heated fluid into subterranean formations.
2. Information Disclosure Statement
The following disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art, inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness and inasmuch as a growing attitude appears to require citation of material, which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also, the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments which may be subsequent in time or priority.
Particularly the recent history of steam injection for enhanced recovery of petroleum or oil deposits furnishes an eloquent example of the extremes to which man will go in his quest for the optimum solution, when confronted with a generally perceived insurmountable impasse in a potentially promising and highly rewarding path of progress.
In this respect, there are in the United States and elsewhere in the world a large number of huge petroleum or oil deposits (hereinafter generically referred to as "oil" or "oil deposit") which cannot be released without intensive thermal stimulation thereof. Various types of thermal stimulation, the perceived objective of which is to raise the temperature of the oil formation to lower the oil viscosity, and enhance oil flow, have heretofore been proposed. These include electric or hot water heaters, gas burners, in situ combustion, hot water or steam injection, and employment of miscible phase displacement fluid, such as carbon dioxide. Of these approaches, the steam injection system appears to be the most promising, though currently beset by various limitations and problems.
Straightforward steam injection systems generate steam above ground and pipe it down the borehole toward the subterranean formation. In practice, the feasibility of such conventional systems ends at relatively low depths, since the steam rapidly loses heat and quality as it travels down the borehole. Even the insulation of steam delivery pipes entails practical depth and cost limitations.
Affordability of such conventional methods is also impaired by the high feedwater quality required of such systems, if higher efficiency is desired. Since feedwater of required purity is seldom available at drilling sites, feedwater treatment is an important part of steam generation. Accordingly, the cost of feedwater, either from a pure source or after suitable treatment, is high to the extent of being unaffordable in most cases.
In consequence, conventional systems frequently employ part of the unboiled water in conjunction with the generated steam as a flushing agent for preventing clogging and similar drawbacks of untreated or poorly treated feedwater. Steam qualities of 80%, with 20% of the piped compound being water, are thus rather frequent. In practice this, of course, still worsens the situation, since the water component does not have the latent heat possessed by steam for the desired recovery enhancement.
Failing further progress in the originally promising direction, in situ generation of steam in the borehole has been tried, particularly in the hope of substantially increasing thereby feasible borehole depths of enhancement.
In terms of historical perspective, such proposals may be viewed in a sense as attempts to miniaturize and to displace a surface boiler way down into the borehole. In practice, that approach, however, has raised serious problems of fuel, combustion air or oxygen supply to, and combustion product removal from, the requisite large depths. Since pumping air at high pressures and over long distances requires equipment of immense size and operating energy, it has been proposed to pipe liquid oxygen, so as to avoid at least the pumping of the useless nitrogen component in natural air. However, this has just further increased equipment and operating costs.
In this respect, even systems which avoid use of high-pressure air compressors, frequently produce unacceptable environmental pollution due to the venting of gases spent in the downhole steam generator.
Thermochemical energy transport systems attempt to avoid problems of the latter type by employing a reversible, catalytically-controlled reaction, particularly the CO.sub.2 --CH.sub.4 reforming methanation chemical cycle, as already known from the solar energy exploitation field.
In one of these systems, an open cycle is employed in which the carbon dioxide and methane developed in the methanation reaction in the downhole catalytic converter/ heat exchanger are not recycled to the reservoir, as in the other system, but are instead discharged into the subterranean formation along with the steam generated in the heat exchanger. This, of course, results in a continuous loss of the carbon monoxide and hydrogen working fluid, and also raises an environmental question as to the effect of the methanation reaction products on the subterranean formation and deposits into which they are discharged.
In either case, whether the methanation reaction or combustion product is recycled or discharged, the heat exchanger in which the steam is formed is followed by the steam expansion chamber which is usual in such downhole systems, before the steam is applied to the subterranean formation.
To date, such reversible, catalytically-controlled reactions have been proposed for the solar energy field and are believed to require considerable research and experience for use in subterranean deposit recovery.
Other methods tried so far include miscible slug and gas processes in which liquid hydrocarbons or gases are injected into the subterranean deposit. In micellar solution flooding, surfactants, colloids or electrolytes are injected, while polymer flooding injects polysaccharide, polyacrylamides or other polymers.
In practice, such methods and their materials are very expensive and of doubtful value, as presently conceived.
Against this state of the art, a peculiar type of in situ combustion which has all the trappings of an act of desparation, appears almost as a relatively attractive solution to the prior-art worker. In particular, that kind of thermal recovery essentially consists of setting the oil reservoir on fire with the aid of injected air or oxygen. The fire in the well and undergound reservoir causes the lighter fractions of oil ahead of the flame to vaporize, leaving a heavy residual coke or carbon deposit as fuel for the reservoir burning process. Vaporized light components and steam formed by combustion are carried forward through the deposit until they condense upon contacting cooler portions of the reservoir. Accordingly, the term forward combustion is often applied to this kind of in situ process to signify that the flame front is advancing the the same direction as the air movement, that is, from the injection well to the producing well.
There also is a reverse combustion in which the ignition is started in the well that is supposed to be the producer. For that kind of process to work at all, the reservoir need to have high air permeability. As in fires on the surface of the earth, simultaneous or alternate injection of water into the conflagration is often indicated.
Disadvantages of such a draconic approach are exposure of the well and the producing equipment to severe damage by heat and corrosion, contamination of the oil and the subterranean formation, and an inherently inefficient operation.
Even worse in this respect is the proposal of Clarence I. Justheim, who in his U.S. Pat. No. 3,237,689, issued Mar. 1, 1966 suggested use of a nuclear reactor for distillation of underground deposits of solid carbonaceous materials in situ. Justheim in particular suggested heat transfer from the nuclear reactor to a circulating molten metal, such as liquid sodium, which, in turn, would transfer heat to a secondary heat-transfer medium via an intermediate heat exchanger. Secondary heat-transfer media mentioned by Justheim include a liquid or a gas or a combination of both, such as water, air, or superheated steam. Obviously, Justheim added the inherent dangers of a nuclear reaction and liquid sodium cycle to the above mentioned inherent dangers and disadvantages of conventional systems.
An oil recovery process utilizing air and superheated steam is disclosed in U.S. Pat. No. 4,083,404, by Joseph C. Allen, issued Apr. 11, 1978. Allen proposed turning the injection well into a superheater by the use of a steam generator, a steam boiler, and an air compressor, injecting superheated steam and air under pressure into the formation whereby in situ combustion is initiated for driving petroleum in the formation toward a production well. This proposal appears to partake of the above mentioned disadvantages of conventional steam generation and pressurized air injection.